1. Field Description of the Invention
The present invention relates to an exhaust-gas purifier for use in methanol-fueled diesel engines.
2. Description of Related Art
A methanol-fueled diesel engine ejects exhaust-gas containing organic substances such as formaldehyde/methanol and the like which are generated by incomplete combustion. Among those organic substances, particularly unfavorable is formaldehyde; it not only smells bad, but also is harmful to human health. Accordingly, direct discharge thereof into open air should be avoided. In the prior art, measures have been taken to decompose the objectionable organic substances including formaldehyde by oxidation prior to discharge using noble metals such as platinum, palladium, cobalt, nickel, and the like as catalysts. This method, however, requires heating the noble metal catalysts to high temperatures of 200.degree. C. or above to bring the catalysts to a sufficiently activated state. Accordingly, the catalysts are of little or no effect in cases where the exhaust-gas temperature is lower than 100.degree. C., such as in the idling period at the very starting of a cold engine. More disadvantageously, formaldehyde and incompletely combusted methanol are particularly heavily discharged at lower engine temperatures, such as at idling temperatures, with the result that the gas exhausted during the idling time is heavily polluted despite the treatment with the aforementioned catalysts.
With a view to solve the aforementioned problems inherent in the treatment with a catalyst, there are disclosed in Unexamined Published Utility Model Applications Sho-62-5820 and Sho-62-10223, arrangement designs comprising adsorbents set upstream of catalytic equipment so that the formaldehyde, methanol, and the like ejected at the engine start-up may be adsorbed until the catalyst comes to show its effect. As the temperature of the system increases, formaldehyde and the like, thus adsorbed on the adsorbents are desorbed but also with the increasing temperatures, the catalyst is sufficiently heated to readily decompose the organic substances. In this way it is possible to avoid direct discharge of an exhaust-gas of high pollution levels.
In the designs aforementioned, examples of the useful adsorbents disclosed include alumina, porous glass, active carbon, silica gel, and the like.
In exhaust-gas treatment apparatus using adsorbents in combination with catalysts, however, it is requisite that the adsorbents possess high retention capacity of the organic substances until the catalysts become sufficiently active, so that the organic substances including formaldehyde may not be ejected into the atmosphere. Larger adsorption capacity can be obtained, simply, by increasing the amount of the adsorbent, but there is always a restriction concerning the installation space for the adsorbent. It is preferred that the adsorbents occupy as small a space as possible. Accordingly, in the present invention it is preferred that the adsorbents be those having higher adsorption capacity per unit volume at low temperature at which the catalysts have no effect. It is further required that, at higher temperatures, the adsorbent gradually reduces its adsorption power and desorb the captured organic substances such as formaldehyde, so that the adsorbent may be used repeatedly in the next idling period.
It has been found that the adsorbents of the prior art exhaust-gas purifiers were deficient in these regards. Notably, an object of the present invention is to provide a more compact adsorbent and an improved purification of the exhaust-gas during the engine idling period.
Another object of the present invention is to provide an adsorbent more efficient in its adsorption-desorption characteristics on components of a methanol-fueled engine exhaust-gas, and to further provide a high-performance exhaust-gas purifier using the same adsorbent.